758 Bashford Dean Memorial Volume 



specimens (compare Text-figures 8 and 9, page 668, with Figures 83 and 84, plate VII). 

 Carman's key, which depends mainly on the positions of certain of the fins, was perhaps 

 not intended to apply to such young specimens. Changes in the spacing of the fins may 

 be brought about by differential growth. 



EXTERNAL AND INTERNAL GILL-FILAMENTS 



To the list of embryos of Heterodontus japonicus, already described, that bear ex- 

 ternal gill-filaments, one must add those embryos represented by Figures 66, 68, 70, 72 and 

 74, plate VI. In the last-named figure the external gill-filaments are very profuse. 



So far as one may judge from the series of embryos portrayed in Dean's drawings, 

 marked individual variations in the degree of development of the external gill-filaments of 

 Heterodontus japonicus are rare. To be sure, the embryo pictured in Figure 36, plate III, 

 is entirely lacking in external filaments; whereas in another embryo of approximately the 

 same general stage (Figure 81, plate VII) the external filaments attain their maximum 

 development. But it is probable that the condition shown in Figure 36, plate III, is 

 exceptional. If this embryo were left out, the remaining series (including those embryos, 

 already noted, which are not represented by drawings) would show a fairly regular 

 gradation in the development and regression of the external gill-filaments. 



The latest member of this series of embryos showing external gills is the one repre- 

 sented in Figure 38, plate III. Therefore the external gill-filaments of Heterodontus japonv 

 cus are not known to persist to such an advanced stage of general development as they 

 do in Chlamydoselachus (Cudger, 1940, pages 629-630 and plate VI). Of Chlamydosela- 

 chus, a female specimen 614 mm. long and a male specimen 538 mm. long are portrayed with 

 short external gills. These specimens had attained the adult form, but were not full-grown 

 and were probably not sexually mature. The reproductive organs of a 1398-mm. female 

 Chlamydoselachus dissected by me (Smith, 1937, Text-figure 85) were decidedly imma- 

 ture, and in a female Chlamydoselachus 1550 mm. long (ibid., Text-figure 86) they were just 

 approaching maturity. In these specimens, as well as in the two sexually mature females 

 dissected by me, the gill-filaments did not show externally with the gill-flaps closed. 



The question arises, what is the relation of the external gill-filaments of the embryo 

 to the permanent gill-filaments of the adult? Of Chlamydoselachus, Gudger (1940, p. 639) 

 writes: "these so-called external gills of the frilled shark are nothing but precociously 

 overgrown permanent gills, which later on shorten until but a bare remnant shows beyond 

 the gill-opening." I have been able to examine gills of Heterodontus in critical stages of 

 their development and to observe that the external gill-filaments are not fundamentally 

 different from the rudiments of the permanent filaments, but are essentially the same 

 structures lengthened distally. It is better to begin with the adult stage and to trace 

 the history of the gill-filaments backward. 



I have had no adult specimen of H. japonicus, but I have examined the gill-filaments 

 in an adult H. quoyi. Here, the filaments are short and deep-set, so that the gill-flaps must 

 be pried well apart before one can observe the filaments with a lens. The fundamental 



